To merge calendars you must export one calendar out of iCal and then re-import it into another calendar of your choice. This can be repeated for all the calendars you wish to merge.
1. Click on the calendar on the left hand side of iCal highlighting it's name and then going to File Export > Export and saving the resulting calendar file to a logical location.
2. Double-click that resulting file and a window in iCal will pop-up asking you where you want the events to be imported -- choose the calendar where you want these events added.
3. Delete the original calendar you exported out in step # 1 from iCal so you don't see duplicates.